1. Technical Field
The present invention relates to megasonic cleaning systems having chemically inert resonators and a piezoelectric crystal and more particularly to a system in which the crystal is bonded directly to the cleaning tank using indium.
2. Background Information
It is well-known that sound waves in the frequency range of 0.4 to 2.0 megahertz (MHZ) can be transmitted into liquids and used to clean particulate matter from damage sensitive substrates. Since this frequency range is predominantly near the megahertz range, the cleaning process is commonly referred to as megasonic cleaning. Among the items that can be cleaned with this process are semiconductor wafers in various stages of the semiconductor device manufacturing process, disk drive media, flat panel displays and other sensitive substrates.
Megasonic acoustic energy is generally created by exciting a crystal with radio frequency AC voltage. The acoustical energy generated by the crystal is passed through an energy transmitting member and into the cleaning fluid. Frequently, the energy transmitting member is a wall of the vessel that holds the cleaning fluid. The crystal and its related components are referred to as a megasonic transducer. For example, U.S. Pat. No. 5,355,048, discloses a megasonic transducer comprised of a piezoelectric crystal attached to a quartz window by several attachment layers. The megasonic transducer operates at approximately 850 KHz. Similarly, U.S. Pat. No. 4,804,007 discloses a megasonic transducer in which energy transmitting members comprised of quartz, sapphire, boron nitride, stainless steel or tantalum are glued to a piezoelectric crystal using epoxy.
It is also known that piezoelectric crystals can be bonded to certain materials using indium. For example, U.S. Pat. No. 3,590,467 discloses a method for bonding a piezoelectric crystal to a delay medium using indium where the delay medium comprises materials such as glasses, fused silica and glass ceramic.
A problem with megasonic transducers of the prior art is that the acoustic power that can be generated by the megasonic transducer in the cleaning solution is limited to about 10 watts per cm2 of active piezoelectric surface without supplying additional cooling to the transducer. For this reason, most megasonic power sources have their output limited, require liquid or forced air cooling or are designed for a fixed output to the piezoelectric transducer or transducers. Typically, fixed output systems are limited to powers of 7-8 watts/cm2. This limits the amount of energy that can be transmitted to the cleaning solution. If more power is applied to the transducer, the crystal can heat up to the point where it becomes less effective at transmitting energy into the cleaning solution. This is caused either by nearing the maximum operating temperature of the crystal or, more often, by reaching the failure temperature of the material used to attach the crystal to the energy transmitting means.
Another problem with prior art cleaning systems that utilize megasonic transducers, is that there is no practical way of replacing a defective transducer once the transducer has been attached to the cleaning system. This means that users have to incur large expenses to replace defective transducers, for example by purchasing a whole new cleaning vessel.
Briefly, the present invention is a megasonic cleaning system comprised of a one-piece cleaning tank, one or more piezoelectric crystals and an indium layer for attaching the piezoelectric crystal (or crystals) to the cleaning tank. The tank comprises a material selected from the group consisting of quartz, sapphire, silicon carbide, silicon nitride, ceramics and stainless steel. The piezoelectric crystal (or crystals) is attached directly to the sides or bottom of the tank.
The piezoelectric crystal (or crystals) is capable of generating acoustic energy in the frequency range of 0.4 to 2.0 MHz when power is applied to the crystal. The attachment layer is comprised of indium and is positioned between the tank and the piezoelectric crystal so as to attach the piezoelectric crystal to the tank. A first adhesion layer comprised of chromium, copper and nickel is positioned in contact with a surface of the piezoelectric crystal. A first wetting layer comprised of silver is positioned between the first adhesion layer and the bonding layer for helping the bonding layer bond to the first adhesion layer.
A combination layer is applied to the region of the tank to which the crystal is to be attached. The combination layer helps the indium layer bond to the tank and preferably comprises a silver conductive emulsion (paste) that is applied to the tank using a screen print process.
Alternatively, the combination layer can be replaced by a second adhesion layer and a second wetting layer. The second adhesion layer is comprised of chromium, copper and nickel and is positioned in contact with a surface of the tank. The second wetting layer is comprised of silver and is positioned between the second adhesion layer and the bonding layer for helping the bonding layer bond to the second adhesion layer.